selftests/bpf: Measure bpf_loop verifier performance

This patch tests bpf_loop in pyperf and strobemeta, and measures the
verifier performance of replacing the traditional for loop
with bpf_loop.

The results are as follows:

~strobemeta~

Baseline
    verification time 6808200 usec
    stack depth 496
    processed 554252 insns (limit 1000000) max_states_per_insn 16
    total_states 15878 peak_states 13489  mark_read 3110
    #192 verif_scale_strobemeta:OK (unrolled loop)

Using bpf_loop
    verification time 31589 usec
    stack depth 96+400
    processed 1513 insns (limit 1000000) max_states_per_insn 2
    total_states 106 peak_states 106 mark_read 60
    #193 verif_scale_strobemeta_bpf_loop:OK

~pyperf600~

Baseline
    verification time 29702486 usec
    stack depth 368
    processed 626838 insns (limit 1000000) max_states_per_insn 7
    total_states 30368 peak_states 30279 mark_read 748
    #182 verif_scale_pyperf600:OK (unrolled loop)

Using bpf_loop
    verification time 148488 usec
    stack depth 320+40
    processed 10518 insns (limit 1000000) max_states_per_insn 10
    total_states 705 peak_states 517 mark_read 38
    #183 verif_scale_pyperf600_bpf_loop:OK

Using the bpf_loop helper led to approximately a 99% decrease
in the verification time and in the number of instructions.

Signed-off-by: Joanne Koong <[email protected]>
Signed-off-by: Alexei Starovoitov <[email protected]>
Acked-by: Andrii Nakryiko <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

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